Intravenously injected nanoparticles (NPs) hold great promise for clinical diagnostic and therapeutic applications. A critical issue in their implementation is in their in vivo biodistribution which leads to inefficiency in targeting approaches resulting in the accumulation of NPs in untargeted (healthy) organs. Our long-term goal is to elucidate the mechanisms of in vivo biodistribution for NPs using a gold NP (GNP) model system. The specific hypothesis is that the in vivo biodistribution of GNPs is dictated by their interactions (or lack thereof) with blood components. This hypothesis is based on the observations that: 1) blood is the first medium of interaction in the body for intravenously injected NPs;2) in vitro studies show that NPs interact with serum proteins (including complement factors), cause platelet aggregation and red blood cell hemolysis;3) our preliminary data shows that the presence of a biomolecule on the surface of the NPs increase their blood retention time. Based on these observations, the experimental focus of this project is on the NP interactions with the blood com ponents. The specific aims are to: (1) Characterize the blood-GNP interactions in vitro. We will study the interactions of GNPs with both the plasma and cellular components. The role of the immune system will be tested indirectly by evaluating the complement system activation in plasma. The interactions of GNPs with the cellular components (RBCs and white blood cells-WBCs) will be evaluated using histology, and the subcellular localization of GNPs will be identified using transmission electron microscopy (TEM). (2) Characterize the blood-GNP interactions in vivo and their impact on biodistribution. We will evpplication proposed by a aluate the role of the compi ement system in the biodistribution of GNPs by conducting experiments in both normal and complement deficient mice. The blood-GNP interactions and their correlation with tissue distribution will be evaluated at various ti me points. The effect of the presence of a tu mor on the blood-GNP interactions will be evaluated by com paring them in normal and human prostate cancer tum or bearing m ice. Public Health Relevance: Given that several nanoparticles are being evaluated in c linical settings for the treatment of cancer, the outcomes of the proposed research will be timely and important. The realization of the proposed research goals will significantly advance the field of nanoparticle biodistributi on by providing new information on the underlying mechanisms that govern their biodistribution. The results of this study will facilitate future research aimed at the design of nanoparticles that can avoid accumulation in healthy organs thereby increasing the delivery effi ciency and reducing the toxicity in clinical settings.